In recent years, the industrial utilization of porous glasses as adsorbing agents, microcarrier supports, separation films, optical materials, and the like has been highly anticipated. In particular, porous glasses have a wide utilization range as optical members because of a characteristic of low refractive index.
As for a method for manufacturing a porous glass relatively easily, a method taking advantage of a phase separation phenomenon has been mentioned. A typical example of a base material for the porous glass exhibiting the phase separation phenomenon is borosilicate glass made from silicon oxide, boron oxide, an alkali metal oxide, and the like. In production, the phase separation phenomenon is induced by a heat treatment in which a molded borosilicate glass is held at a constant temperature (hereafter referred to as a phase separation treatment), and a non-silicon oxide rich phase, which is a soluble component, is eluted through etching with an acid solution. The skeleton constituting the thus produced porous glass is primarily silicon oxide. The skeleton diameter, the hole diameter, and the porosity of the porous glass have influences on the reflectance and the refractive index of the light.
NPL 1 discloses a configuration in which the porosity of a porous glass simple substance is controlled in etching in such a way that elution of a non-silicon oxide rich phase is allowed to become insufficient partly and, thereby, the refractive index increases from the surface toward the inside. Consequently, reflection at a porous glass surface is reduced.
Meanwhile, PTL 1 discloses a method for forming a porous glass layer on a base member. Specifically, a film containing borosilicate glass (phase-separable glass) is formed on a base member by a printing method, and a porous glass layer is formed on the base member by a phase separation treatment and an etching treatment.
In the case where several micrometers of porous glass layer is formed on the base member as described in PTL 1, when light is incident on the porous glass surface, the light reflected at the porous glass surface interferes with the light reflected at the interface between the base member and the porous glass, so that a ripple (interference fringe) occurs.